It is known to use ion-sensitive field effect transistor (ISFET) devices to measure the concentration of ions in a solution. However, ISFET devices frequently need to be recalibrated since pH electrodes tend to change in resistive properties over time. GB2451596 to Kahn teaches an ISFET device whose surface is modified with semiconductor sensors and do not require calibration for a few days after a first use. However, Kahn's ISFET device still requires recalibration after a week's time.
WO2010142773 to Thalhammer teaches an ISFET device that calibrates itself by providing multiple identical ISFET transistors, where one or two of the ISFET transistors might be constantly exposed to a calibration substance which would allow the system to estimate how other pH electrodes might change in resistive properties over time. Thalhammer's system is still imperfect, since not all ISFET transistor electrodes are completely identical, and they cannot all change in resistive properties in the same manner over time.
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Thus, there is still a need for improved systems and methods for calibrating an ion meter.